Digitalizing an image and determining the location of a certain amount of electrons on a given surface is at present time carried out using, for instance, a "resistive anode", which is a metallic film of a square shape, mounted on a ceramic surface (see FIG. 3) with 4 terminals, one on each corner, connected to current amplifiers. At the arrival of a certain amount of electrons to a given place in the surface, these create an electric current that is displaced, upon being attracted by electric fields to the 4 terminals. The amount of electrons that are gathered at each corner depends on the exact point of arrival of the electrons, that is to say, the closest terminal captures the greatest number and the farthest one, the least amount of them (see FIG. 4a). The generated current has a very low value and it is necessary to be amplified and processed by means of special circuits which are relatively expensive. Already processed signals are utilized to determine, on a coordinate XY axis system, the position of the geometric center of the electron cloud (see FIG. 4b).
One of the disadvantages of the use of the resistive anode, in cases like the one stated above, is that when 2 or more electron clouds are present at the same time, the result in that the geometric center of the clouds can be false (as it is in FIG. 4c). Other disadvantages are, for instance, its high cost; the necessary electronics that must be added for its adecuate functioning; the slow system response upon arrival of the electrons; furthermore, the exact location of the center of the electron cloud depends on the precise measurement of the small currents that travel to the corners, which are easily contaminated with noise, giving non-dependable results.